1. Field of the Invention
The present invention relates to a heater cable in combination with a lead cable. More particularly, the invention relates to a wiring pattern which is capable of minimizing the generation of leak magnetic field from the heater cable.
2. Description of the Prior Art
A conventional sheet-like heating appliance such as an electric blanket or an electric carpet is fabricated by wiring a heater cable 40 and a sensor cable 50 in a combination in a carpet fabric material; for example, the heater cable 40 comprising, as shown in FIG. 15, a heating element wire 32 wound in a spiral at intervals of a given distance on the outer surface of a polyester core thread 31, a fusing layer 33 of nylon or polyester provided over the heating element wire 32, a short-circuit wire 34 wound in a spiral on the outer surface of the fusing layer 33, and a protective coating 35 of polyvinyl chloride (PVC) provided over the short-circuit wire 34, and the sensor cable 50 comprising, as shown in FIG. 16, a signal wire 42 wound in a spiral on the outer surface of a polyester core thread 41, a fusing layer 43 provided over the signal wire 42, another signal wire 44 wound in a spiral on the outer surface of the fusing layer 43, and a protective coating 45 provided over the signal wire 44.
In such a conventional sheet-like heating appliance when turned on, the heating element wire 32 of the heater cable 40 only is supplied with a flow of current.
The fusing layer 33 and 43 have a so-called fusing function (fuse action) that when the heater cable 40 is overheated with a sensor wire 50 on the sheet-like heating body malfunctioning and its temperature locally rises up close to the melting point of a resin material of the fusing layer 33 and 43 (referred to as a fusing layer material hereinafter), the fusing layer material is softened or fused to short-circuit between the heating element wire 32 and the short-circuit wire 34, and the signal wires 42 and 44, and to disconnect the power supply. More particularly, the fusing layer 33 and 43 are designed to act as finally a safety circuit for protecting the sheet-like heating body from being overheated.
The polyamide resin, such as nylon-12, which is used in the fusing layer 33 and 43 have acceptable levels of molding formability, mechanical properties, and thermal properties required for the fusing layer material.
When the heater cable 40 of the conventional appliance arranged in a known pattern is energized, the current which flows in the heater cable 40 may generate a leak magnetic field. Consequently, high frequencies of the leak magnetic field develop a magnetic noise which gives adverse effects over the other electric appliances. It is now said that low frequencies may also affect the human body. Therefore, those problems are particularly emphasized in the conventional sheet-like heating appliances, namely electric blankets and carpets, for warming the human body.
The present invention has been schemed for solving the above problems of the prior art and its object is to provide a wiring pattern which is capable of diminishing the leak magnetic field from the heater cable, as preferably applicable to a sheet-like heating appliance such an electric blanket or an electric carpet.
As a first feature of the present invention, a heater cable in combination with a lead cable, in which the heater cable comprising a heating element wire wound in a spiral on the outer surface of a core thread, a fusing layer coated over the heating element wire, a signal wire wound in a spiral on the outer surface of the fusing layer and a protective coating provided over the signal wire, the lead cable comprising a conductive wire covered at its outer surface with an insulating coating, and the heater cable and the lead cable are formed such a pattern that the heater cable and the lead cable are spaced by a predetermined distance L from each other and are capable to be supplied with opposite flows of current is provided.
In the heater cable in combination with the lead cable according to the first feature of the present invention, the two opposite flows of current introduced into the heater cable 10 and the lead cable 20 respectively permit their induced intensities of magnetic field to be offset by each other, hence diminishing the leak magnetic field.
As a second feature of the present invention is a heater cable in combination with a lead cable, wherein the distance L is not greater than 10 mm.
In the heater cable in combination with the lead cable according to the second feature of the present invention, the distance L between the heater cable 10 and the lead cable 20 which are uniformly spaced from each other in the wiring pattern is set not greater than 10 mm, thus effectively diminishing the leak magnetic field. It is desired that the above mentioned fusing layer comprises copolymer polyester resin which has the following characteristics (a) and/or (b) and (c) to (f).
(a) The angle xcex1 between the baseline and a tangent to the heat absorption peak of a Differential Scanning Colorimetry (DSC) curve defined by a Japanese Industrial Standard (JIS) K7121 (ISO 3146) method is 90xc2x0 to 120xc2x0.
(b) The angle xcex1xe2x80x2 between the baseline and a tangent to the shift portion of a ThermoMechanical Analysis (TMA) curve determined in an expansion mode by a thermal-mechanical measuring method is 90xc2x0 to 100xc2x0.
(c) The melting point is 160xc2x0 C. to 210xc2x0 C.
(d) The reduction viscosity is not lower than 0.5 dl/g.
(e) The moisture absorption is not higher than 1.0% at a temperature of 20xc2x0 C. and a relative moisture rate of 65%.
(f) The hardness measured by a JIS K7215 (ISO 868) method is Hardness Degree (HDD) 40 to 74.
Also, it is desired that the anti-heat deterioration of the resin of the fusing layer stand for 500 hours at a temperature of 140xc2x0 C.
The composition of the above mentioned copolymer polyester resin is now explained in more detail. One of its acid components is aromatic dicarboxylic acid which may be selected from terephthalic acid, isophthalic acid, 2-6-naphthalenedicaboxylic acid, biphenyldicarboxylic acid, so on and their ester. Similarly, its aliphatic dicarboxylic acid component having 2 to 20 carbon atoms may be selected from succinic acid, glutaric acid, adipic acid, azelaic acid, sebacic acid, dodecanedionic acid, dimer acid, so on and their ester. Its alicyclic dicarboxylic acid component may be selected from 1.4-cyclohexanedicarboxylic acid, so on and its ester. Its hydroxy-carboxylic acid component may be selected form p-hydroxy-aromatic acid, so on and its ester.
Its glycol component is aliphatic glycol which may be selected from 1,4-butanediol, 1,3-butanediol, 1,2-butanediol, ethylene glycol, propylene glycol, 1,2-propanediol, 1,3-propanediol, 1,5-pentadiol, 3-metyl-1,5-pentadiol, 1,9-nonanediol, 1,6-hexanediol, doracanedionic acid, neopentyl glycol, neopentyl glycol hydroxy pivalate, 2-ethyl-2-butylpropanediol, and bisphenol-A-ethylene-oxide or -propylene-oxide derivative, so on.
Other copolymer components than the above mentioned are glycol such as polyethylene glycol, polypropylene glycol, polytetramethylene glycol, so on, xcex5-caprolactone, hyroxycapronic acid, polycaprolactone, and tricyclodecanedimethylol, so on.
The above materials may arbitrarily be combined in a desired combination or ratio without departing from the characteristics (a) to (f) defined above to produce a type of the copolymer polyester resin according to the present invention. The copolymer polyester resin of the present invention may be added, if desired, with anti-oxidation agents, stabilizers, anti-copper agents, non-organic fillers, nucleating agents, surface activators, anti-static agents, flame-retardants, plasticizers, and thickening agents, so on.
Thermoplastic resins are generally classified into crystalline polymer and non-crystalline polymer depending on their molecular structures. Such polymers have a wide range of melting points in a given temperature range based on the motion of a main chain of the polymers and show no sharp phase shift compared with non-organic compounds and metallic substances, thus determining no definite melting points. This allows the resin material of the fusing layer in a heater cable to commonly employ polyamide resins (nylon-11 and nylon-12) which have comparatively definite melting points. Polyamide resins are however less favorable in the moisture absorption than the other resins. We, the inventors, had been engaged to focus on copolymer polyester resins to develop a novel material which is not related to the disadvantages of polyamide resins. After closely examining the resin composition and the behavior of softening and melting, we invented a novel copolymer polyester resin featuring particular relationship between the DSC and TMA characteristics and the fusing characteristic essential for a heater cable. The action of the characteristics (a) to (f) of the present invention will now be described in detail.
Having the characteristic (a) that the angle xcex1 between the baseline and a tangent to the heat absorption peak of a DSC curve is 90xc2x0 to 120xc2x0 (preferably, 90xc2x0 to 110xc2x0 ), the fusing layer provides a sharp thermal fusing function. However, when the angle a exceeds 120xc2x0, a range of the softening fluidity of the resin will be increased hence declining the thermal fusing function.
Having the characteristic (b) that the angle xcex1xe2x80x2 between the baseline and a tangent to the shift portion of a TMA curve is 90xc2x0 to 100xc2x0 (preferably, 90xc2x0 to 95xc2x0), the fusing layer provides as a sharp thermal fusing function as of the characteristic (a). However, when the angle xcex1xe2x80x2 exceeds 100xc2x0, a range of the softening fluidity of the resin will be increased hence declining the thermal fusing function. This may cause actual measurements of the fusing temperature to vary more or less.
Also, as the thermal behavior is governed by the characteristics (a) and (b), the melting point is practically limited to a definite range (160 to 210xc2x0 C.). This allows the fusing temperature to be arbitrarily changed within the range. Accordingly, the selection of the fusing temperature can be made from a wider range than that of polyamide resin, hence permitting the fabrication of more practically useful heater cable.
As specified by the character (c), the copolymer polyester resin has a melting point ranging from 160xc2x0 C. to 210xc2x0 C. (preferably, from 170xc2x0 C. to 200xc2x0 C.) and can thus be used favorably as the material of the fusing layer. If the melting point is set to lower than 160xc2x0 C, the heater cable may often be heated close to 160xc2x0 C. locally depending on its using condition and the temperature will rise easily to its limit level of overheat protection determined by the design criterion. This discourages the reliability of the heater cable as the commercial product. When the melting point is set to higher than 210xc2x0 C., the fusing temperature of the fusing layer becomes too high, i.e. the surface temperature of the product is too high, and will hardly satisfy applicable safe standards.
As specified by the characteristic (d), the copolymer polyester resin has a reduction viscosity of not lower than 0.5dl/g (preferably, not lower than 0.7dl/g) and can favorably be used as the material of the fusing layer. If the reduction viscosity is set to lower than 0. 5dl/g, the delivery of the resin during the tubing extrusion will be unstable thus failing to form a uniform thickness of the fusing layer. This causes the heater cable to have poor mechanical properties, for example, less in the strength and the extensibility, thus shortening the operating life.
As specified by the characteristic (e), the copolymer polyester resin has a moisture absorption of not higher than 1.0% (preferably, not higher than 0.8%) at a temperature of 20xc2x0 C. and a relative moisture rate of 65% and can favorably be used as the material of the fusing layer. If the moisture absorption exceeds 1.0%, the resin may easily be affected by an ambient condition (moisture).
The moisture absorption of nylon-11 and nylon-12 which are commonly used is substantially 1.3% (cited from xe2x80x9cChemical Fiber IIxe2x80x9d, p. 406, by Hiroshi Matsuzaki and Kenji Oshina, Maruzen Publishing in Japan) at a temperature of 20xc2x0 C. and a relative moisture rate of 65%, which is much higher than 1.0%. In fact, such nylon-11 and nylon-12 are still employed with their disadvantage on the moisture absorption being compensated since no appropriate materials for the fusing layer are so far provided which can replace the nylon-11 and nylon-12.
A polyester resin comprising terephthalic acid and ethylene glycol is known which has a small level of water (moisture) absorption and is good in electric characteristics. This resin is favorable in mass production and overwhelmingly accepted as one of the most common-use resins. This is now used as a cable core for a heating conductor in a heater cable but not as the material as the fusing layer. One of the reasons is that the melting point of the resin is as high as 256 to 265xc2x0 C. (cited from xe2x80x9cSynthetic Polymer Vxe2x80x9d by Shunsuke Murahashi) and not favorable for use as a thermal fuse in view of the safety requirements.
As specified by the characteristic (f), the copolymer polyester resin has a hardness of HDD 40 to 74 (preferably, HDD 50 to 70) and can favorably used as the material of the fusing layer. If the hardness is smaller than HDD 40, the fusing layer will be too soft when extrusion processed. This causes the short-circuit signal line to be sunk down into the fusing layer when wound over, whereby the fusing temperature will vary more or less. If the hardness exceeds HDD 74, the fusing layer will be too hard when extrusion processed. This causes the heater cable to be declined in the flexibility, hence making wiring works difficult in the production of electric blankets or carpets.